This invention relates to a novel trimerization catalyst system and to a process for the trimerization of isocyanates in the presence of this catalyst system. The catalyst system comprises (A) a lithium compound selected from the group consisting of: (i) lithium salts of aliphatic or aromatic mono- or dicarboxylic acids, (ii) lithium salts of hydroxyl group containing compounds having from 1 to 3 hydroxyl groups per compound, wherein the hydroxyl groups are directly attached to an aromatic ring, and (iii) lithium hydroxide; (B) an allophanate catalyst; and (C) an organic compound containing at least one hydroxyl group.
The trimerization of isocyanates to form polyisocyanurates is well known in the art. Trimerization catalysts described in the prior art include alkali carboxylates as described in DE-OS 3,219,608, basic alkali metal salts complexed with acyclic organic compounds as described in U.S. Pat. No. 4,379,905, basic alkali metal salts complexed with crown ethers as described in U.S. Pat. No. 4,487,928, and combinations of tertiary amines with specific quaternary ammonium salts as described in U.S. Pat. No. 3,954,684.
Catalysts described in U.S. Pat. Nos. 4,632,785 and 4,540,781 comprise alkali metal salts or quaternary ammonium salts of carboxylic acids of the formulas ##STR1## wherein R.sup.1 is alkyl having from 2 to 8 carbon atoms, R.sup.2 is a highly branched alkyl having from 3 to 8 carbon atoms, R.sup.3 is selected from the group consisting of hydrogen, alkyl, and aryl, R.sup.4 is selected from the group consisting of alkyl, aryl, aralkyl, and cycloalkyl, R.sup.5 is independently selected from aryl, and M.sup.+ is a cation selected from the group consisting of alkali metal cations and quaternary ammonium cations of a specific formula.
Canadian Patent Application 2,113,890 relates to trimer catalyst systems for aliphatic and aromatic isocyanates. The trimer catalyst systems of this earlier application comprise (A) a lithium compound selected from the group consisting of: (i) lithium salts of aliphatic or aromatic monocarboxylic or dicarboxylic acids, (ii) lithium salts of hydroxyl group containing compounds containing from 1 to 3 hydroxyl groups per compound, wherein the hydroxyl groups are directly attached to an aromatic ring, and (iii) lithium hydroxide; and (B) an organic compound containing at least one hydroxyl group.
Advantages of the presently claimed catalyst systems in comparison to the catalyst systems of Canadian Patent Application 2,113,890 include the fact that the reaction is difficult to control using these earlier catalyst systems because the rate of reaction increases with the time, and thus results in runaway reactions. The novel catalyst systems of the present application, which contain an allophanate catalyst, provide steady and predictable rates of reaction which do not change significantly as the length of reaction time increases, i.e., these reactions are smooth and "well-behaved". In addition, catalyst systems of the present application work well at lower temperatures (i.e., .ltoreq.100.degree. C.) than the catalyst systems of my earlier copending application.